In fluid pumping applications using positive displacement pumps, the pump creates flow. A restriction to the flow created by a system downstream from the pump creates pump back pressure. High back pressure is problematic because it is known to cause various mechanical failures including accelerated pump wear due excessive pump loading, pump failure, and leaks or failure of downstream system components of the pump, such as hoses, hose fittings, filters canisters, and manifolds.
To protect against over pressurization in these systems, a mechanical high pressure bypass or pressure regulating valve is commonly employed. These valves usually consist of a ball bearing closing an inlet by being pressed against an inlet circumscribing machined surface by a spring sitting on an adjustable seat. Moving the adjustable seat towards the ball bearing compresses the spring thereby increasing the fluid pressure required at the inlet to dislodge the ball. Decreasing the spring pressure will allow fluid to escape through the inlet at lower pressures.
Accordingly, the venting of fluid from the system at a desired set point is dependent on the proper and successful operation of the bypass valve to protect the pump and other components from exposure to pressures above the system design. Notwithstanding, the mechanical high pressure bypass valves in some applications can be difficult to tune.
Additionally, it is possible for instabilities to arise within more complex systems causing the bypass valve to oscillate open and close. The resulting pressure fluctuations are generally unacceptable. Accordingly, bypass valves are subject to wear, degraded performance, and ultimately failure. A failed valve will usually compromise the system, causing unacceptably high bypass flow as a result of the valve opening or leaking at low pressure under otherwise normal conditions thereby requiring replacement.
Furthermore, high pressure relief valves, associated fittings, and tank return hosing can be a significant portion of total system cost. The percentage will vary with system design and complexity, but the point holds true: cost is an important consideration for pump systems on, for example mobile engine fuels systems.
In systems trying to achieve a constant flow design, the systems must be sized large enough and are generally oversized to accommodate the system downstream flow demand and any planned flow through the high pressure bypass.
Moreover, constant speed pumps are not constant flow for many types of positive displacement pumps. Volumetric efficiency varies as back pressure changes. For example, higher back pressure causes more leakage and recirculation within the pump leading to a change in flow which can be large depending on the pressure range and pump construction. This again results in a general use of an oversized system since a constant speed pump must be sized large enough to supply the system load when the system restriction (back pressure) is at its maximum design point.
For the foregoing reasons, there is a need to ameliorate or overcome one or more of the significant shortcomings delineated hereinabove.